The present invention relates to a device for controlling a working arm of a working machine. More specifically, the invention relates to a device for controlling a working arm, which is capable of suitably controlling a pressure that generates and is confined in a hydraulic cylinder provided in a working arm device of a working machine such as a front shovel device of a hydraulic shovel, a lift arm device of a wheel loader, and the like.
With reference to FIG. 8, a hydraulic shovel as generally designated at 2, which is a typical working machine equipped with a working arm device, includes a lower running body 4 and an upper turning body 6 mounted on the lower running body 4 to freely turn on a pivot 5. The upper turning body 6 is provided with a front shovel device 8 that is a working arm device. The front shovel device 8 includes a boom 10 mounted on the upper turning body 6 to freely turn in the up and down directions, a boom actuation cylinder 12 interposed between the upper turning body 6 and the boom 10, an arm 14 mounted on an end of the boom 10 to freely turn, an arm actuation cylinder 16 interposed between the arm 14 and the boom 10, an attachment 18 attached to an end of the arm 14 to freely turn, such as a breaker, and an attachment actuation cylinder 20 interposed between the attachment 18 and the boom 10.
With reference to FIG. 8 together with FIG. 9, the hydraulic shovel 2 is equipped with the above-mentioned hydraulic actuators and a hydraulic pressure control unit for controlling the actuation of the hydraulic actuators such as the attachment 18, a turning motor 22 and a pair of running motors 24a, 24b. The hydraulic pressure control unit includes hydraulic pumps 28a, 28b driven by a motor 26, and a control valve 30 that controls a blow-out fluid to supply it to the hydraulic actuators. The control valve 30 includes plural direction control valves corresponding to each of the hydraulic actuators. A direction control valve 32 is connected to the arm actuation cylinder 16, a direction control valve 34 is connected to the attachment actuation cylinder 20, and a direction control valve 36 is connected to the boom actuation cylinder 12. The direction control valve 32 is operated by a pilot fluid output from an arm operation remote control valve 38a of a pilot operation means 38, and the direction control valve 34 is operated by a pilot fluid output from an attachment operation remote control valve 38b of the operation means 38. To a fluid passage 40a on the rod side, which is a pressurized fluid feed/drain circuit connecting the attachment actuation cylinder 20 to the direction control valve 34, there are connected a cylinder relief valve 42a for limiting the hydraulic pressure in the fluid passage 40a and a check valve 44a that permits flow of the actuation fluid from a tank 46 into the fluid passage 40a. Similarly, a cylinder relief valve 42b and a check valve 44b are connected to a fluid passage 40b of the head side.
With further reference to FIGS. 10 and 11, when the attachment actuation cylinder 20 is extended and contracted by the operation of the attachment operation remote control valve 38b, the attachment 18 pivots on an end of the arm 14 between a position where the attachment 18 is pulled toward the side of the upper turning body 6 shown in FIG. 10 and a position where the attachment 18 is separated far away from the upper turning body 6 shown in FIG. 11. The arm 14 is provided with stoppers 14a and 14b for limiting the turning ends of the attachment 18. When the arm actuation cylinder 16 is extended and contracted by the operation of the arm operation remote control valve 38a, the arm 14 pivots on an end of the boom 10 as a center between a position (solid lines) where the attachment 18 is pulled toward the upper turning body 6 shown in FIGS. 10 and 11 and a position (two-dot chain lines) where the attachment 18 is separated far away from the upper turning body 6 shown in FIGS. 10 and 11. Thus, the attachment 18 can be brought to any desired position to meet the work.
Referring to FIGS. 10 and 11 illustrating the working states of the working arm device 8, FIG. 10 shows a state where the attachment actuation cylinder 20 is contracted to fully turn the attachment 18 until it comes into contact with the stopper 14a to maintain the attachment actuation cylinder 20 in a state of being not in operation and the arm actuation cylinder 16 is extended to turn the arm 14 in a direction of an arrow U to lift it up. FIG. 11 shows a state where the attachment actuation cylinder 20 is extended to bring the attachment 18 into contact with the other stopper 14b to maintain the attachment actuation cylinder 20 in a state of being extended and the arm actuation cylinder 16 is contracted to turn the arm 14 in a direction of an arrow D to lower it down.
In the state of FIG. 10, as the arm actuation cylinder 16 is extended, the overall length of the attachment actuation cylinder 20 in its state of being maintained extends from X1 to X2 due to a difference in the positions of mounting the arm 14 and of mounting the attachment actuation cylinder 20 on the boom 10. The extension of the attachment actuation cylinder 20 in this state of being maintained is accomplished by releasing the actuation fluid in a fluid chamber 20a on the rod side, that is compressed with the progress of extension, into a tank 46 under a set pressure of the cylinder relief valve 42a, and filling up a fluid chamber 20b on the head side with the actuation fluid from the tank 46 via a check valve 44a. 
In the state shown in FIG. 11, as the arm actuation cylinder 16 is contracted, the overall length of the attachment actuation cylinder 20 in its state of being maintained contracts from X3 to X4. The actuation fluid in the fluid chamber 20b on the head side, that is compressed with the progress of contraction, is released into the tank 46 under a set pressure of the cylinder relief valve 42b and the fluid chamber 20a on the rod side is filled up with the actuation fluid from the tank 46 via a check valve 44b. 
The above-mentioned conventional working arm device involves problems that must be solved as described below.
That is, as the attachment 18 comes into contact with the stopper 14a or 14b thereby to bring the attachment actuation cylinder 20 into a state of being not in operation and the arm actuation cylinder 16 is extended or contracted, the cylinder relieve valve 42a or 42b works to release the actuation fluid confined in the attachment actuation cylinder 20. Therefore, the hydraulic force for extending or contracting the arm actuation cylinder 16 is raised by an amount of energy for releasing the actuation fluid from the relief valve 42a or 42b. Hence, this amount becomes a loss of energy.
In addition, the above energy released with a high pressure turns into heat and causes the actuation fluid to be overheated. Further, the actuation speed of the arm actuation cylinder 16 decreases due to an increase in the load resistance at the time of extension or contraction. Depending upon the weight of the attachment 18 and positions of turning of the attachment 18 and the arm 14, further, the arm actuation cylinder 16 comes into a halt due to the load resistance. Consequently, the workability of the working arm device 8 is deteriorated.
Though this problem can be improved if the set pressure of the cylinder relief valves 42a and 42b is lowered, the lowering of the set pressure results in a decrease in a maximum output of the attachment actuation cylinder 20 limited by the set pressure and in a decrease in the force for holding the attachment 18 to the arm 14 thereby to weaken the operation force of the attachment 18, with the consequence that the attachment 18 moves due to the external force during the working or the attachment 18 moves due to its own weight, making it difficult to carry out the operation appropriately.
The present invention has been done in view of the above-mentioned facts, and its technical assignment is to provide a device for controlling a working arm of a working machine, which is capable of releasing the pressure of the actuation fluid confined in the actuation cylinder of the working arm device at a predetermined pressure lower than the set pressure without changing the set pressure of the cylinder relief valve and in which the opening/closing of the cylinder relief valve is controlled.
In order to solve the above-mentioned technical assignment, the present invention provides a device for controlling a working arm of a working machine comprising:
a releasing means for releasing a pressurized fluid feed/drain circuit of an actuation cylinder of a working arm device at a predetermined pressure lower than a set pressure of a cylinder relief valve provided for the feed/drain circuit; and
a control means for controlling the opening/closing of the releasing means.
The releasing means is operated by the control means to meet the actuation state of the working arm device, and the pressure that generates and is confined in the actuation cylinder is released at a pressure lower than the set pressure of the cylinder relief valve.
According to a preferred embodiment, the working arm device includes a turnable arm member, a turning member mounted on the arm member so as to freely turn, an arm member actuation cylinder for actuating the arm member, a turning member actuation cylinder for actuating the turning member, and stoppers for limiting the turning ends of the turning member, and the control means operates the releasing means in a state where the turning member is in contact with the stopper, the turning member actuation cylinder is not in operation and the arm member actuation cylinder is in operation.
The pressure confined in the turning member actuation cylinder that elevates due to the contact of the turning member of the working arm device with the stopper, is released at a pressure lower than the set pressure of the cylinder relief valve.
The control means includes a contact detector means for detecting the contact of the turning member with the stopper and an operation detector means for detecting the operation of the turning member actuation cylinder, and operates the releasing means by using an operation signal for operating the arm member actuation cylinder based on output signals from the contact detector means and the operation detector means.
The releasing means is operated by a signal that operates the arm member actuation cylinder while the contact detector means detects the contacting state and the operation detector means detects the non-operating state.
According to a preferred embodiment, the releasing means includes a direction change-over valve connected to the pressurized fluid feed/drain circuit and a relief valve connected to the outlet port of the direction change-over valve, and changes over the direction change-over valve by using the control means.
The direction change-over valve is changed over by the control means, and the confined pressure is released at a low pressure through the relief valve that is set to a predetermined low pressure.
According to a further embodiment, the releasing means includes a set pressure-varying relief valve for varying the set pressure in response to an external signal and an adjustment means for adjusting the external signal, and controls the external signal by using the control means.
The external signal for setting the set pressure of the set pressure-varying relief valve to a predetermined low pressure is adjusted by the adjustment means and is controlled by the control means.
The arm member actuation cylinder is operated by a pilot operation means, and uses a pilot fluid output from the operation means as the operation signal.
The pilot fluid output from the operation means is fed as an operation signal to the releasing means at the time of operating the arm member actuation cylinder, and the releasing member is actuated being interlocked to the operation of the arm member actuation cylinder.